Metal objects used in machinery and equipment are often required to be manufactured to narrow dimensional tolerance and with high dimensional stability. However, it is often difficult to produce very large metal objects, such as, for example, a cylinder, to a high degree of dimensional stability, when the cross-sectional dimension of the cylinder is increased. This problem becomes increasingly worse as the thickness of the cylinder wall becomes thinner. In particular, handling the cylinder during manufacture is very difficult because such handling during processing and/or storage often causes defects.
One method of dealing with this problem is to form an electroform by an electroforming process on a mandrel, remove the electroform from the mandrel and then fill the electroform with a filling material such as foam. Unfortunately, however, as one increases the cross-sectional dimension of the electroform and/or the complexity of the circumference, one must increase the thickness of the electroform and support it during the foam filling operation, or the desired dimensional characteristics may be lost. This, however, increases the material cost, the processing time, the weight, and may require an additional processing step.
U.S. Pat. No. 4,473,516 to Hunerberg discloses a method and apparatus for injecting molded plastic articles having a solid outer surface and porous interior core with a molten mixture of chemically reactive foaming agent and a thermoplastic resin. The mold cavity is filled with the plastic mixture and subsequently an activator additive is introduced into the mixture which reacts with the foaming agent and facilitates cellular expansion within the core of the molded article. The result is a molded body with a solid unfoamed skin which accurately replicates the surface of the mold and a cellular inner core.
U.S. Pat. No. 4,627,894 to Monnier discloses a method of manufacturing a molded plastic body which is covered on its exterior with a metal layer. A metallic layer is deposited on the inner surface of a mold by an electrolytic method before the plastic material is introduced. A thermoplastic or thermosetting material is then introduced into the mold. After hardening of the plastic, the material removal from the mold is effected by applying thermal treatment.
U.S. Pat. No. 3,464,898 to Norris discloses an electroforming process for forming complex hollow metal articles in which a conductive material is deposited on the surface of a fine porosity plastic mandrel and a layer of metal is electrodeposited onto the conductive material in an electroforming bath. Finally, separation of the mandrel from the metal article, for example by use of a solvent or volatilization of the plastic mandrel, produces a metal article which is easily shaped and which maintains dimensional accuracy.
U.S. Pat. No. 4,781,799 to Herbert, Jr. et al. discloses an electroforming apparatus and process of forming electroformed articles which consists of an elongated electroform mandrel having two mating ends (segments) which can mate with each other during the electroforming process and which each have a circumferential, electrically conductive electroforming surface. During the electroforming process, electroforming a metal layer on the electroforming surface of each segment establishes a gap between each metal layer and the underlying segment. Subsequently, the removal of each metal layer from the underlying segment by sliding the metal layer axially along the underlying segment in which the end of the metal layer adjacent to the mating end of the underlying segment produces an article having a smooth round outer edge and improved dimensional requirements.
U.S. Pat. No. 4,664,758 to Grey discloses an electroforming process which incorporates an elongated electroforming mandrel core, applying a uniform coating of a molten, inert, inorganic, homogeneous electrically conductive metal or metal alloy to the mandrel core (the metal having a melting point and surface tension less than that of the mandrel core) and immersing the mandrel/metal composite in an electroforming bath. Next, an electroformed metal layer having a melting point greater than the metal or metal alloy is deposited on the coating and the coating is melted. Finally, the electroformed metal layer is removed from the mandrel core producing an electroformed article having the required dimensions and surface characteristics.